Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation.
Photosensitive optoelectronic devices convert electromagnetic radiation into an electrical signal or electricity. Solar cells, also called photovoltaic (“PV”) devices, are a type of photosensitive optoelectronic device that is specifically used to generate electrical power. Photoconductor cells are a type of photosensitive optoelectronic device that are used in conjunction with signal detection circuitry which monitors the resistance of the device to detect changes due to absorbed light. Photodetectors, which may receive an applied bias voltage, are a type of photosensitive optoelectronic device that are used in conjunction with current detecting circuits which measures the current generated when the photodetector is exposed to electromagnetic radiation.
These three classes of photosensitive optoelectronic devices may be distinguished according to whether a rectifying junction as defined below is present and also according to whether the device is operated with an external applied voltage, also known as a bias or bias voltage. A photoconductor cell does not have a rectifying junction and is normally operated with a bias. A PV device has at least one rectifying junction and is operated with no bias. A photodetector has at least one rectifying junction and is usually but not always operated with a bias.
As used herein, the term “rectifying” denotes, inter alia, that an interface has an asymmetric conduction characteristic, i.e., the interface supports electronic charge transport preferably in one direction. The term “photoconductive” generally relates to the process in which electromagnetic radiant energy is absorbed and thereby converted to excitation energy of electric charge carriers so that the carriers can conduct (i.e., transport) electric charge in a material. The term “photoconductive material” refers to semiconductor materials which are utilized for their property of absorbing electromagnetic radiation to generate electric charge carriers. There may be intervening layers, unless it is specified that the first layer is “in physical contact with” or “in direct contact with” the second layer.
When electromagnetic radiation of an appropriate energy is incident upon an organic semiconductor material, a photon can be absorbed to produce an excited state. In organic photoconductive materials, the generated excited molecular state is generally believed to be an “exciton,” i.e., an electron-hole pair in a bound state which is transported as a quasi-particle. An exciton can have an appreciable life-time before geminate recombination (“quenching”), which refers to the original electron and hole recombining with each other (as opposed to recombination with holes or electrons from other pairs). To produce a photocurrent, the electron and hole forming the exciton are typically separated at a rectifying junction.
Excitons also form in inorganic semiconductors. However, the Coulomb interaction between electrons and holes in inorganic materials is weaker than in organic materials, such that the electron and hole may disassociate in inorganic materials before reaching a rectifying junction.
In the case of photosensitive devices, the rectifying junction is referred to as a photovoltaic heterojunction. To produce internally generated electric fields at the photovoltaic heterojunction which occupy a substantial volume, the usual method is to juxtapose two layers of material with appropriately selected semi-conductive properties, especially with respect to their distribution of energy states.
Types of organic photovoltaic heterojunctions include a donor-acceptor heterojunction formed at an interface of a donor material and an acceptor material, and a Schottky-barrier heterojunction formed at the interface of a organic photoconductive material and a metal. Types of inorganic photovoltaic heterojunctions include a p-n heterojunction formed at an interface of a p-type doped material and an n-type doped material, and a Schottky-barrier heterojunction formed at the interface of an inorganic photoconductive material and a metal. A photovoltaic heterojunction can also be formed at an interface between an inorganic material and an organic material.
In organic photovoltaic heterojunctions, the materials forming the heterojunctions have been denoted as being donors or acceptors. In the context of organic materials, the terms “donor” and “acceptor” refer to the relative positions of the Highest Occupied Molecular Orbital (“HOMO”) and Lowest Unoccupied Molecular Orbital (“LUMO”) energy levels of two contacting but different organic materials. If the LUMO energy level of one material in contact with another is lower, then that material is an acceptor. Otherwise it is a donor. It is energetically favorable, in the absence of an external bias, for electrons at a donor-acceptor junction to move into the acceptor material.
Organic semiconductors and insulators may have additional discrete molecular orbitals below the HOMO and above the LUMO, typically identified as HOMO−1, HOMO−2, LUMO+1, LUMO+2, etc.
The use of “donor” and “acceptor” with organic materials has a different meaning than with inorganic materials. In the context of organic materials, the terms “donor” and “acceptor” refer to the relative positions of the HOMO and LUMO energy levels of two contacting but different materials. This is in contrast to the use of these terms in the inorganic context, where “donor” and “acceptor” may refer to types of dopant atoms that may be used to create inorganic n- and p- types layers, respectively.
One common feature of semiconductors and insulators is a “band gap.” The band gap is the energy difference between the highest energy level filled with electrons and the lowest energy level that is ordinarily empty. In an inorganic semiconductor or inorganic insulator, this energy difference is the difference between the valence band edge EV (top of the valence band) and the conduction band edge EC (bottom of the conduction band). In an organic semiconductor or organic insulator, this energy difference is the difference between the HOMO and the LUMO. The band gap of a pure material is devoid of energy states where electrons and holes can exist. The only available carriers for conduction are the electrons and holes which have enough energy to be excited across the band gap. In general, semiconductors have a relatively small band gap in comparison to insulators.
In terms of an energy band/level model, excitation of a valence band electron in an inorganic semiconductor into the conduction band creates carriers; that is, electrons are charge carriers when on the conduction band side of the band gap, and holes are charge carriers when on the valence band side of the band gap. Likewise, for organic semiconductors, electrons are charge carriers when on the unoccupied molecular orbital side of the band gap, and holes are charge carriers when on the occupied molecular orbital side of the band gap. Put more succinctly, electrons are carriers above the band gap, and holes are carriers below the band gap.
As used herein, a first energy level is “above,” “greater than,” or “higher than” a second energy level if the first energy level is closer to the vacuum energy level. A higher HOMO energy level corresponds to an ionization potential (“IP”) having a smaller absolute energy relative to a vacuum level. Similarly, a higher LUMO energy level corresponds to an electron affinity (“EA”) having a smaller absolute energy relative to vacuum level. On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material.
As is the convention with energy band diagrams, it is energetically favorable for electrons to move to a lower energy level, whereas it is energetically favorable for holes to move to a higher energy level (which is a lower potential energy for a hole, but is higher relative to an energy band diagram). Put more succinctly, electrons fall down whereas holes fall up.
Carrier mobility is a significant property in inorganic and organic semiconductors. Mobility measures the ease with which a charge carrier can move through a conducting material in response to an electric field. In the context of photosensitive devices, a material that conducts preferentially by electrons due to a high electron mobility may be referred to as an electron transport material. A material that conducts preferentially by holes due to a high hole mobility may be referred to as a hole transport material. A layer that conducts preferentially by electrons, due to mobility and/or position in the device, may be referred to as an electron transport layer (“ETL”). A layer that conducts preferentially by holes, due to mobility and/or position in the device, may be referred to as a hole transport layer (“HTL”). Preferably, but not necessarily, an acceptor material (organic) and an n-type material (inorganic) are electron transport materials; and a donor material (organic) and a p-type material (inorganic) are hole transport materials. In comparison to semiconductors, insulators generally provide poor carrier mobility.
As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be a fluorescent or phosphorescent small molecule emitter. A dendrimer may be a “small molecule.” In general, a small molecule has a defined chemical formula with a molecular weight that is the same from molecule to molecule, whereas a polymer has a defined chemical formula with a molecular weight that may vary from molecule to molecule. As used herein, “organic” includes metal complexes of hydrocarbyl and heteroatom-substituted hydrocarbyl ligands.
For additional background explanation and description of the state of the art for organic photosensitive devices, including their general construction, characteristics, materials, and features, U.S. Pat. No. 6,657,378 to Forrest et al., U.S. Pat. No. 6,580,027 to Forrest et al., and U.S. Pat. No. 6,352,777 to Bulovic et al. are incorporated herein by reference.